1. Field of the Invention
The invention relates to an abnormality diagnosis apparatus and an abnormality diagnosis method for a water temperature sensor.
2. Description of the Related Art
In a water temperature sensor for detecting a coolant temperature in an internal combustion engine, in addition to continuous abnormalities such as disconnections, shorts, or the like, intermittent abnormalities such as repetitive switching between a normal and an abnormal state also occur. There is strong demand for the development of technology that can determine whether such intermittent abnormalities are taking place. To address this demand, attention has been focused on sudden changes that occur in a detection value of the water temperature sensor when there is an abnormality, such as in the technology disclosed in Japanese Patent Laid-open Publication No. JP-A-11-173149 (specifically, paragraphs [0051], [0053]). Such sudden changes in the detection value can be used as a basis for making a determination as to whether an intermittent abnormality is occurring in the water temperature sensor.
However, it is not the case that sudden changes occur in the detection value every time there is an intermittent abnormality of the water temperature sensor. Depending on the type of intermittent abnormality, for example, the effect of the abnormality may appear in the form that, there is a difference between (i) a movement of an actual coolant temperature, and (ii) a movement of the detection value of the water temperature sensor, even though the detection value generally changes gradually along with change in the actual coolant temperature. In this situation, no sudden change in the detection value of the water temperature sensor accompanies the abnormality. Accordingly, if abnormality diagnosis of the water temperature sensor is performed in a manner like that disclosed in Japanese Patent Laid-open Publication No. JP-A-11-173149, it is not possible to determine whether an abnormality is occurring in the water temperature sensor.
Further, types of abnormality diagnosis have been disclosed in which it is determined that an abnormality is occurring in the water temperature sensor based on whether or not a detection value of the water temperature sensor has reached a predetermined reference value at a time point when sufficient time has elapsed for warm up to be completed since engine start up began (for example, refer to Japanese Patent Laid-open Publication No. JP-A-11-200940 (specifically, paragraph [0006]).
In this abnormality diagnosis, a comparatively long time period is provided and ensured to allow for completion of warm up. This long time period is provided and ensured so that, even in the case of a very slight abnormality in which the output characteristics of the water temperature sensor only diverge very slightly from a normal state, it is possible to detect that the abnormality is occurring. Thus, if an intermittent abnormality of the water sensor is occurring, the effect of the abnormality will be noticeably apparent in the detection value of the water temperature sensor at the time point when the time necessary for warm up has elapsed. More specifically, the detection value of the water temperature sensor resulting from the abnormality becomes lower than the reference value. As a result, even in the case of types of abnormality which do not cause a sudden change of the detection value of the water temperature sensor, it is possible to determine that an abnormality is occurring based on the fact that the detection value of the water temperature sensor has become lower than the reference value at the time point when the time necessary for warm up has elapsed.
However, in the abnormality diagnosis disclosed in Japanese Patent Laid-open Publication No. JP-A-11-200940, it is necessary to provide and ensure the comparatively long time period until the time necessary for the completion of warm up has elapsed after the beginning of engine start up. As a result, it is not possible to determine whether an abnormality of the water temperature sensor is occurring until this time has elapsed, and thus making the abnormality determination requires a period of time.